Stable mouse monoclonal hybridoma cell lines secreting antibodies against Listeria monocytogenes were produced. Antibodies from two of these cell lines (designated CL2 and CL17) have been partially characterised. The specificities of these antibodies were assessed using indirect immunofluorescence antibody tests and L monocytogenes (166 strains) grown in vitro, other species of Listeria (21 strains), and bacteria from 14 other genera (87 strains). The antibodies were found to be specific for Listeria, and when used in combination, reacted with almost all strains of L monocytogenes. A simple and rapid direct immunofluorescence technique was developed, and the presence of L monocytogenes was shown in necropsy tissue from three patients where listeriosis had been confirmed by isolation of the bacterium. Bacteria were also confirmed using one of these antibodies in necropsy tissue from one further patient in whom listeriosis was suspected, but not confirmed by the cultivation of L monocytogenes.